Trim knob settings

[Saxomohawh]

There is some very good info about the inputs (mic/line) and the Trim knobs written in the "Sticky" section. However, I find some things are a little unclear, or contradictory.
"The Trim Knob generally should be set full right for Low Impedance (LowZ) sources (mics), and full left for high impedance (HiZ) sources (guitars, keyboards, outboard effects returns, pre-amps, etc.)"
But then there is also written (for HiZ sources):
"Start with the trim control at approximately the 1 o’clock position. This is the ‘unity gain’ setting where the machine’s pre-amp neither amplifies nor attenuates the signal"
If so, shouldnt this always be the ideal position (if possible, of course) for both HiZ and LoZ inputs?
Or at least try to avoid going higher than 1 o´clock if not neccesary? To have the knob fully clockwise for mic input would be cranking the preamp to max, isn´t it? That can´t be optimal if you want to achieve highest possible quality, can it?

 

[Mark Richards]

You've asked a good question.

The Trim Knob generally should be set ...full left for high impedance (HiZ) sources

The first section is the most conservative approach to take. It assumes the HiZ source is professional gear having a nominal level +4dBU output and adjustable output gain control. In this case, you would want to use the gain control of the outboard gear to control the signal into the portastudio to optimize signal-to-noise (S/N) ratio and minimize potential distortion.

"Start with the trim control at approximately the 1 o’clock position.

The second section is most applicable when the HiZ source is consumer or non-professional gear having a nominal level -10dBU output, and which may or may not have variable output gain control. In this case you could use the Trim Knob to compensate for a weak output of the source, or to obtain a better S/N ratio for the incoming signal.

Overall, if you look at the Level Diagram in your Owner Manual, on the Input side you'll see that:

• A nominal +4dBU incoming analog signal produces a 0dBFS digital signal. In this case you would want the Trim Knob full left, and you would want to control the incoming signal using the outboard gear.
• A -10dBU incoming analog signal produces (approximately) a -14dBFS digital signal. So in this case it would be ok to use the Trim Knob to increase a weaker incoming signal if the outboard gear doesn't have a gain control or the gain control output level is insufficient.

 

[Saxomohawh]

Thanks, this answers the first part of my question(s). But for LoZ, microphone input, why would it be ideal to have the knob at "max"?

 

[Mark Richards]

for LoZ, microphone input, why would it be ideal to have the knob at "max"?

Again, a very good question.

Typically good microphones are spec'd to have a starting output of -60dBU to -50dBU, so you would want to maximize the incoming signal; thus start with the Trim Knob at the full right position; and then position the mic for optimum recording level.

Looking at the Level Diagram in the Owner Manual:

For a mic with a minimum output of -56dBU, the Trim Knob full right provides 42dB of headroom, so that a mic'd (vocal/insrument) signal coming in at -14dBU produces a -16dBFS digital signal; and there's an additional 14dB of headroom available before reaching 0dBFS.

In practice, professional mics are capable of producing signals that will exceed 0dBFS, depending on placement, vocal/instrument level being mic'ed etc.

The professional way to control a mic's actual output level is to identify the ideal placement location relative to the output volume, timbre, phase issues, etc. of the signal source.

Sometimes, even after doing that, the signal intermittently can be too hot. That's when the Trim Knob of the portastudio gets used to lower the overall level of the incoming mic signal to prevent distortion.

Other boards, and some mics, have -20dB and/or -40dB pads that can be inserted/used to tame hot mic level, but the portastudios obviously don't.